A DC coupled amplifier fed by an RF detector

Abstract: A DC coupled amplifier (207) fed by an RF detector (205) has greater dynamic range capabilities than techniques in the prior art. An illustrative embodiment of the present invention advantageously comprises a low-offset voltage, low-offset current differential amplifier (207) fed by an RF detector (205) having a fifth lead (223) from the reference voltage node (231) within the RF detector.

Description: This invention relates to apparatus for detecting a signal. As shown in FIG.1, RF detector 105 feeds single-ended amplifier 107 a DC coupled video signal. Depending upon the mechanical construction of the apparatus comprising RF detector 105 and single-ended amplifier 109, the return current from RF detector 105 can take several paths. Bulkhead grounding through coaxial connectors usually takes most of the current, but some always flows through the sneak paths, represented by ground resistors 151, causing a voltage error at the reference ground lead 121. This voltage, while perhaps only a millivolt, can easily drown the signal voltage in applications that require a wide dynamic range. The result is that part of the voltage error propagates through RF detector 107 and is superimposed on top of the signal on lead 125. US-A-4791380 discloses a detector circuit for radio-frequency signals that is formed of a pair of matched diodes disposed on a common substrate which is heated by a temperature responsive feedback circuit to stabilize the temperature of the substrate and the diodes. A capacitor is connected to one or both of the diodes to form a detector of the detector circuit. A bias circuit provides equal bias currents to the two diodes, and an amplifier is connected to the two diodes to extract voltages associated with the bias currents, and to subtract these voltages in a differential manner so as to exclude the effect of bias voltage from an output signal of the amplifier. The amplifier is also coupled to an output terminal of the detector to isolate the detector from subsequent stages of signal processing, as well as to amplify detected signals. The diodes are each, preferably, Schottky diodes for improved dynamic range of signal detection. This detector circuit suffers from the voltage errors at a reference ground lead due to current flowing through parasitic resistances of sneak paths. US-A-4689576 discloses an apparatus and method for linearizing a non-linear device. This apparatus consists of a detecting device for detecting a signal when received, a feedback device for providing a feedback signal, an operational amplifier, and a bias means. These devices are coupled to provide an output that is linearly dependent upon the input to the circuit and not upon the amplitude of the signal received. According to this invention there is provided apparatus as claimed in claim 1. The present invention provides a mechanism for detecting a DC coupled RF signal while avoiding many of the costs and restrictions associated with prior techniques. Specifically, embodiments of the present invention are generally less susceptible to ground loops than techniques in the prior art. These results are obtained in an illustrative embodiment of the present invention which comprises an RF detector and a differential amplifier wherein a first differential input to the differential amplifier is connected to the signal output of the RF detector and a second differential input is connected to a reference node in the RF detector. Brief Description of the Drawing FIG. 1 shows a schematic diagram of a DC coupled amplifier as in the prior art. FIG.2 shows a schematic diagram of an illustrative embodiment of the present invention. FIG. 3 shows an illustrative differential amplifier suitable for incorporation in the illustrative embodiment shown in FIG. 2. Detailed Description FIG. 2 depicts a block diagram of an illustrative embodiment of the present invention, which detects and amplifies a DC coupled RF signal. The illustrative embodiment advantageously comprises: power supply 201, signal source 203, RF detector 205, differential amplifier 207 and signal destination 209. Signal source 203 is advantageously capable of amplitude modulating an RF signal and of outputting the modulated signal onto lead 215. RF detector 205 advantageously comprises: diode 261, inductor 262, capacitor 263, and reference node 231. Diode 261 is preferably a zero-based diode or a tunnel diode between lead 215 and lead 219. Inductor 262 advantageously lies between lead 215 and reference node 231. Capacitor 263 advantageously lies between lead 219 and reference node 231. Resistors R 1 , R 2 , and those designated by 151 are parasitic resistors caused by the mechanical connections, and by the bulk resistivity of the metals. Typical values of resistance are in the range of 20 to 1000 micro-ohms. Differential amplifier 207 is preferably a video amplifier (i.e., which amplifies down to 0 Hz) and is characterized by a low offset voltage and a low offset current. A schematic diagram of an illustrative differential amplifier suitable for incorporation into the illustrative embodiment is shown in FIG. 3. The non-inverting input of differential amplifier 207 is advantageously connected to the signal output of RF detector 205 via lead 219. The inverting input of differential amplifier 207 is advantageously connected to reference node 231 via lead 223. Since there is almost no current in this lead, any resistance associated with the lead is of no consequence. The non-inverting input of differential amplifier 207 can, however, alternately be connected to reference node 231 when the inverting input is connected to the signal output of RF detector 205. The output of differential amplifier 207 is fed into the signal destination via lead 225. With this configuration, voltage offsets from the parasitic resistances are canceled. FIG. 3 shows a schematic diagram of a differential amplifier characterized by a low offset voltage and a low offset current. This configuration is often referred to as a composite amplifier. Differential amplifier 207 is a composite amplifier comprising amplifier 301 and amplifier 303. Amplifier 301 is preferably a low-speed, high-precision DC amplifier such as Analog Device's OP-227 and amplifier 303 is preferably a high-speed, low-precision videoamplifier such as Comlinear Corporation's CLC-401. The composite amplifier has the precision of amplifier 301 and the speed of amplifier 303. Both amplifiers operate in conjunction to force lead 309 to a virtual ground at all frequencies including DC. The non-inverting input 223 has much less bandwidth than the inverting input.

Claims: Apparatus for detecting a signal, comprising an RF detector (205) having a signal-in-lead (215) and a ground-in lead (217) for receiving an input signal, and a signal-out lead (219) and a ground-out lead (221) for outputting an output signal, the ground-out lead (221) being separated from the ground-in lead (217) by parasitic resistances, CHARACTERIZED BY: a ground-reference point (231) disposed between the ground-in lead (217) and the ground-out lead (221); and a differential amplifier (207) for amplifying the output signal, comprising: a first differential input connected to said signal-out lead (219), a second differential input, of different polarity from said first differential input, connected to said ground-reference point (231), a ground lead connected to said ground-out lead (221), and an amplifier output. Apparatus as claimed in claim 1 wherein said differential amplifier comprises a composite amplifier. The apparatus as claimed in claim 2 wherein said composite amplifier comprises: a low-speed, high-precision DC amplifier (301); and a high-speed videoamplifier (303). Apparatus as claimed in claim 3 wherein: said precision DC amplifier comprises an output lead (305); said high-speed videoamplifier comprises an inverting lead (311) and a non-inverting lead (305); and said output lead is connected to said non-inverting lead.

Inventors: ARNTZ BERNARD JAMES, ARNTZ, BERNARD JAMES
Applicants: AT & T CORP, AT&T CORP.
Application year: 1994
Main classification: H03F3
IPC codes: H03F3, H03D1, H04B3